Answer;
Electron microscope
Explanation;
Electron microscopy (EM) has long been used in the discovery and description of viruses. Viruses have a size of roughly 1/100 of bacteria (or smaller), so they are too small to be seen in light microscopy.
Viruses are considerably smaller, as they are merely nucleic acid surrounded by a protein shell as opposed to an entire living cell. Viruses typically range from 20-400 nm, which means an electron microscope is needed to see them.